The existence of the histamine H3 receptor has been known for several years and the receptor is of current interest for the development of new medicaments (see eg Stark, H.; Schlicker, E.; Schunack, W., Drugs Fut. 1996, 21, 507–520; Leurs, R.; Timmerman, H.; Vollinga, R. C., Progress in Drug Research 1995, 45, 107–165). Recently, the human histamine H3 receptor has been cloned, cf Lovenberg, T. W. et al, Molecular Pharmacology, June 1999, 55, 1101–1107. The histamine H3 receptor is a presynaptic autoreceptor located both in the central and the peripheral nervous system, the skin and in organs such as the lung, the intestine, probably the spleen and the gastrointestinal tract. Recent evidence suggests that the H3 receptor show intrinsic, constitutive activity, in vitro as well as in vivo (ie it is active in the absence of an agonist; see eg Morisset et al., Nature 2000, 408, 860–864). Compounds acting as inverse agonists can inhibit this activity. The histamine H3 receptor has been demonstrated to regulate the release of histamine and also of other neurotransmitters such as serotonin and acetylcholine. A histamine H3 receptor antagonist or inverse agonist would therefore be expected to increase the release of these neurotransmitters in the brain. A histamine H3 receptor agonist, on the contrary, leads to an inhibition of the biosynthesis of histamine and an inhibition of the release of histamine and also of other neurotransmitters such as serotonin and acetylcholine. These findings suggest that histamine H3 receptor agonists, inverse agonists and antagonists could be important mediators of neuronal activity. Accordingly, the histamine H3 receptor is an important target for new therapeutics.
Piperazines similar to the compounds of the present invention have previously been prepared, and their biological properties have been investigated.
JP 57175168, JP 01035827 and WO 81/02421 disclose the compound:

JP 63026754 discloses the compound:

Nishi et al. (Chem.Pharm.Bull.; 31; 3; 1983; 852–860) disclose the compound:

Tiwari et al. (Drug Des. Discovery 1995; 12(3); 249–58) and Meanwell et al. (J. Med. Chem. 1992; 35(14); 2688–96 disclose the compound:

WO 95/00512 discloses the compound:

WO 00/51984 discloses indole-containing piperazine derivatives.
DE 19621221 discloses the compound:

U.S. Pat. No. 2,724,713 discloses the following compound:

Dauzonne et al. (J. Med. Chem. Chim. Ther.; 30; 1; 1995; 53–60) disclose the compound:

Vejdelek et al. (Res. Inst. Pharm. Biochem. Commun.; 48; 10; 1983; 2977–88) disclose the following compound as a potential antitussive:

Brown et al. (J. Am. Chem. Soc.; 119; 14; 1997; 3288–3295) disclose the following compound:

Gayral et al. (Arzneim.-Forsch.; 45; 10; 1995; 1122–1127) disclose the following compound:

WO 00/76970 discloses the compound:

EP 0 203 743 discloses the compound:

WO 92/02498 and Valenta et al. (Collect. Czech. Chem. Commun. 1990; 55(6); 1613–29) disclose the compound:

DE 2360362 discloses the compound:

CA59:13982a discloses the compound:

DE 2141634 discloses the compound:

However, these references neither disclose nor suggest that these substituted piperazines may have a histamine H3 receptor antagonistic or agonistic activity.
Several publications disclose the preparation and use of histamine H3 agonists and antagonists. Most of these are imidazole derivatives (see eg Stark et al., Drugs of the Future 1996, 21, 507–520; Tozer, Kalinddjian, Expert Opinion on Therapeutic Patents, 2000, 10, 1045–1055). However, recently some imidazole-free ligands of the histamine H3 receptor have been described (see eg Walczynski et al., Arch. Pharm. Pharm. Med. Chem. 1999, 332, 389–398; Linney et al., J. Med. Chem. 2000, 43, 2362–2370; Ganellin et al., Arch. Pharm. Pharm. Med. Chem. 1998, 331, 395–404; Walczynski et al., II Farmaco 1999, 54, 684–694; WO 99/42458, EP 0 978 512, WO 97/17345, U.S. Pat. No. 6,316,475, WO 01/66534, WO 01/74810, WO 01/44191, WO 01/74815, WO 01/74773, WO 01/74813, WO 01/74814 and WO 02/12190.
However, these compounds differ structurally from the present compounds.
In view of the art's interest in histamine H3 receptor agonists, inverse agonists and antagonists, novel compounds which interact with the histamine H3 receptor would be a highly desirable contribution to the art. The present invention provides such a contribution to the art being based on the finding that a novel class of substituted piperazines has a high and specific affinity to the histamine H3 receptor.
Due to their interaction with the histamine H3 receptor, the present compounds are useful in the treatment of a wide range of conditions and disorders in which an interaction with the histamine H3 receptor is beneficial. Thus, the compounds may find use eg in the treatment of diseases of the central nervous system, the peripheral nervous system, the cardiovascular system, the pulmonary system, the gastrointestinal system and the endocrinological system.